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#!/usr/bin/perl

...
$_ = "#tag1 #tag2 #tag3 #tag1\n";
1 while s/(#\w{2,})\b.*\K\1\b\h*//;
print "keeping only the first one:  $_";

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                  original:  #tag1 #tag2 #tag3 #tag1
 keeping only the last one:  #tag2 #tag3 #tag1
keeping only the first one:  #tag1 #tag2 #tag3